Update ci.yml

Author: jeazhang

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.7, 3.8, 3.9, 3.10]
+        python-version: ['3.7', '3.8', '3.9', '3.10']
 
     steps:
     - uses: actions/checkout@v2